Welded vs Bolted Tanks: A Complex Choice?

In the world of industrial storage, choosing between welded and bolted tanks can be complicated. Both options have their own benefits and drawbacks. The decision often depends on the specific needs of your industry and the type of materials you’re storing.

In this article, we’ll discuss some important factors to consider when deciding which option is best for your project. We warmly invite and value input from others in this industry to comment on this article, as your insights can also aid others in their decision-making process.

Welded Tanks

Welded tanks are typically made of less material than bolted tanks, making them cheaper from a material standpoint. However, assembling a large welded tank is more time-consuming and also requires on-site welding and radiography, which necessitates thorough planning and execution. Therefore, if the cost of assembly (welding) labor is factored in, a welded tank tends to be a more expensive option. In cases where the end-client performs the assembly without considering the labor costs, it may still be the preferred solution. Additionally, the integrity and sealing behavior of a welded tank are superior to those of bolted tanks. Moreover, tank design codes such as API650 and EN14015 have a proven safety record when all specifications are followed.

Bolted Tanks

Bolted tanks, on the other hand, come in different types, each with its own set of advantages and disadvantages. A bolted tank allows for modular sections to be bolted together on-site, enabling significantly smaller and less skilled installation crews to complete the installation in a shorter duration with precise alignment and a perfect fit. An additional benefit is the ability to relocate the tank. Therefore, bolted tanks are an attractive choice for many industries. Some companies claim that the total project savings when opting for a bolted tank instead of a welded tank are around 30%.

Two types of bolted tanks are typically used in the industry: the flange type and the overlapped type.

Bolted Flange Tanks

Bolted flange tanks offer the following key advantages:

1. External Bolts: The bolts are located on the outside, eliminating the need for internal protection against conditions such as corrosion and hydrogen embrittlement.
2. Efficient Installation: The installation process is sped up by requiring fewer but larger bolts.
3. Inspection Ease: The external location of the bolts allows for easy inspection during operation.
4. Rigidity: The flange adds rigidity to the shell panel, simplifying its handling.
5. Tolerance in Fabrication: Misalignment in bolt holes can be easily rectified by reaming. If necessary, the flange can be cut and rewelded, whereas an entire panel would need to be discarded in the case of overlapped types.

However, bolted flange tanks also have their drawbacks:

1. Prying Action: This can increase the risk of bolt failure if the pre-tension is too close to the yield strength of the bolt.
2. Panel Deformation: Welding a large flange to a thin shell can cause panel deformation, especially in stainless steel.
3. Joint Separation: The prying action of the joint can cause separation under hydrostatic liquid load, affecting the sealing behaviour of the joint. Therefore, it is advisable to perform Finite Element Analysis (FEA) on the joint to simulate and verify the prying action. In some tanks with dangerous or toxic content, a small seal weld is sometimes used to ensure a leak-tight joint.

Lapped Type Bolted Tanks

Lapped type bolted tanks are another variant of bolted tanks that come with their own set of pros and cons.

Advantages of Lapped Type Bolted Tanks

1. Less Material: Lapped joints tanks use less material than flanged type.
2. Sealing: If fabrication tolerances are good the sealing of the joint is better than the flange type that is prone to prying action.

Disadvantages of Lapped Type Bolted Tanks

1. Fabrication Tolerance: Lapped joints, if not 100% accurate, can cause major problems for aligning the bolts during assembly.
2. Sealing: Some tank fabricators prefer using bladders instead of gasket types in overlapped bolted tanks. This is because they struggle to achieve a 100% seal using gaskets and therefore prefer the bladder on the inside.
3. Thin Wall Tanks: The AWWA Code allows one to use a very thin wall from a hydrostatic point of view. However, it is very difficult to work with these thin plates, and they tend to buckle at the connection points to the jacking system used.

Several design codes and standards are available that specify the design and construction of bolted tanks. The most common codes used in the industry are API 12B and AWWA D103. However, a limitation of these standards is that they do not cover flange-type connections; they only cover overlapped panel connections. Therefore, Finite Element Analysis is an important tool to obtain the stresses in the flange and bolts and to determine the deformation at the joint, which is crucial for the sealing of the joint.

In conclusion, the choice between welded and bolted tanks, and among different types of bolted tanks, depends on a variety of factors, including the specific requirements of the industry, the nature of the materials stored, and the experience and preferences of the fabricators and end clients. 

What is your preferred choice? Bolted or welded? What problems do you experience in practice with these tanks, and what other considerations should be taken into account?